In recent years, highly precise and fine image quality comparative to silver salt photography can be taken along with rapid progress of recording technology; therefore, ink-jet recording apparatuses for forming images using an ink-jet recording system have been widely used as image forming apparatuses.
Higher speed when forming images and improvement in toughness of obtained images are strongly desired for the ink-jet recording apparatuses. For the countermeasure, in some cases, pigment ink excellent in light resistance and water resistance compared to dye ink has also been used. However, there is a problem with the pigment ink in that ejection failure of the ink may occur when forming images after the ink has not been ejected from recording heads for a while as the pigment ink tends to thicken or solidify due to drying of the ink.
In order to solve the problem, surfactants or polymer dispersants as additives of the ink have been investigated from various aspects for the purpose of stabilizing ejection of the ink for ink-jet recording apparatuses. A water-based ink has also been proposed in which a hydrophilic group is introduced onto a surface of a pigment, for example, and thus the pigment can be stably dispersed in an aqueous medium without dispersant.
In the case of using such an ink, ink ejection is satisfactory when forming images after the ink has not been ejected from recording heads for a while. However, in cases of using recording heads having a small-sized nozzle with an aim to improve image quality, there arises a problem of occurrence of disarray in an ejecting direction of ink droplets; therefore, it is necessary to improve ejection stability of the ink.
Furthermore, when forming images using the pigment ink with ink-jet recording apparatuses, the ink tends to adhere to discharge rollers before the ink has penetrated into recording media when the recording media are discharged through a pair of discharge rollers (offset). In this case, image defects are likely to occur in obtained images due to adhesion to recording media of the ink which has adhered to the discharge rollers. For this reason, a penetrating agent to promote ink infiltration into recording media is often added to the ink in order to suppress occurrence of image defects due to the offset.
However, in cases of including the penetrating agent into the ink, there is also a problem in that colors of ink droplets ejected from recording heads, after the ink was not ejected from recording heads for a while, become lighter colors or transparent.
In this regard, the pigment ink without dispersant described above does not contain a so-called penetrating agent. Accordingly, the problem in that colors of ink droplets become lighter colors or transparent, ejected from recording heads after the ink was not ejected from recording heads for a while, has not been eliminated by using the pigment ink described above in cases of using the pigment ink containing the penetrating agent.